linux-kernel - Re: [PATCH 1/5] prctl: add PR_ISOLATE

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Date:   Tue, 23 Jan 2018 18:07:19 +0100
From:   Dominik Brodowski <linux@...inikbrodowski.net>
To:     Martin Schwidefsky <schwidefsky@...ibm.com>
Cc:     linux-kernel@...r.kernel.org, linux-s390@...r.kernel.org,
        kvm@...r.kernel.org, Heiko Carstens <heiko.carstens@...ibm.com>,
        Christian Borntraeger <borntraeger@...ibm.com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Cornelia Huck <cohuck@...hat.com>,
        David Hildenbrand <david@...hat.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Jon Masters <jcm@...hat.com>,
        Marcus Meissner <meissner@...e.de>,
        Jiri Kosina <jkosina@...e.cz>, w@....eu, keescook@...omium.org
Subject: Re: [PATCH 1/5] prctl: add PR_ISOLATE_BP process control

On Tue, Jan 23, 2018 at 02:07:01PM +0100, Martin Schwidefsky wrote:
> Add the PR_ISOLATE_BP operation to prctl. The effect of the process
> control is to make all branch prediction entries created by the execution
> of the user space code of this task not applicable to kernel code or the
> code of any other task.

What is the rationale for requiring a per-process *opt-in* for this added
protection?

For KPTI on x86, the exact opposite approach is being discussed (see, e.g.
http://lkml.kernel.org/r/1515612500-14505-1-git-send-email-w@1wt.eu ): By
default, play it safe, with KPTI enabled. But for "trusted" processes, one
may opt out using prctrl.

Thanks,
	Dominik